Puf repeats (aka PUM-HD, Pumilio homology domain) are necessary and sufficient for sequence specific RNA binding in fly Pumilio and worm FBF-1 and FBF-2. Both proteins function as translational repressors in early embryonic development by binding sequences in the 3' UTR of target mRNAs (e.g. the nanos response element (NRE) in fly Hunchback mRNA, or the point mutation element (PME) in worm fem-3 mRNA). Other proteins that contain Puf domains are also plausible RNA binding proteins. Puf domains usually occur as a tandem repeat of 8 domains. The Pfam model does not necessarily recognize all 8 repeats in all sequences; some sequences appear to have 5 or 6 repeats on initial analysis, but further analysis suggests the presence of additional divergent repeats. Structures of PUF repeat proteins show they consist of a two helix structure.